1. Field of the Invention
The present invention relates to a band pass filter and related frequency down converter, and more particularly, to a band pass filter and related frequency down converter for enhancing rejecting effect of image frequency.
2. Description of the Prior Art
In a broadcast system, a superheterodyne receiver is the most widespread use receiver, which can execute carrier frequency adjustment (namely select a channel), filtering, and amplifying. In the superheterodyne receiver, signal is received by an antenna, and performed amplifying, RF (radio-frequency) filtering, IF (intermediate frequency) transformation, and finally, via one or more IF amplifying and filtering processes, transformed to a base frequency band for succeeding demodulation. Transforming RF to IF is always influenced by image frequency interference, and may cause some problems.
Please refer to FIG. 1, which is a schematic diagram of a frequency down converter 10 for a superheterodyne receiver according to the prior art. The frequency down converter 10 includes low noise amplifiers 100 and 102, a receiver end 103, an image reject filter 104, a mixer 106, a local oscillator 108, an IF low pass filter 110, and an intermediate frequency amplifier 112. Below is a summary of an operation method of the frequency down converter 10. An RF signal VRF1 is received by an antenna, and enters the frequency down converter 10. The RF signal VRF1 is amplified to an RF signal VRF2 via the low noise amplifiers 100 and 102. Then, the image reject filter 104 receives the RF signal VRF2 via the receiver end 103, and filters out image frequency signals to generate a filtered RF signal VFRF. Finally, the filtered RF signal VFRF transforms to an IF section through the mixer 106, and outputs IF signal VIF via filtering of the IF low pass filter 110 and amplifying of the IF amplifier 112. The image reject filter 104 is used for removing interference of the image frequency. A cause of the image frequency is two input frequencies |fLO±fIF| are both outputted to a frequency fIF through the mixer 106. The frequency fLO is an oscillatory signal frequency of the local oscillator 108, and the frequency fIF is a frequency of the IF signal VIF. Therefore, in the superheterodyne receiver, when a signal of spectrum corresponding to sides of a local oscillating signal goes through the mixer 106, the signals enter the same spectrum, and form an interference signal which lowers a signal to interference ratio, influences a desired received signal, and affects a receiving efficiency of the superheterodyne receiver. For solving a problem of image frequency interference, the most common method is to add a band pass filter in front of the mixer 106, i.e., the image reject filter 104, for filtering out the interference signal before entering the mixer 106, so as to lower the interference.
In order to examine an effect of the frequency down converter 10, there is an important standard which is image frequency rejection ratio defined as a gain between a received frequency and an image frequency. For example, in a satellite frequency down converter, the general standard is 40 db. Besides, a difference of an insertion loss between the received frequency and the image frequency of the image reject filter 104 is the most important parameter for deciding the image frequency rejection ratio of the frequency down converter 10.
There are many methods for realizing the image reject filter 104 according to the prior art, for example, hairpin band pass filter, parallel-coupled line filter, etc. Please refer to FIG. 2, which is a schematic diagram of a hairpin band pass filter 20 according to the prior art. The hairpin band pass filter 20 is a transverse symmetry structure, which includes micro-strip ports IO_a and IO_b, and resonators RSN_1˜RSN_n. The micro-strip ports IO_a and IO_b connect to a front-stage and a rear-stage circuit for receiving and outputting signals. A length of each of the resonators RSN_1˜RSN_n is half of a wavelength corresponding to a desired received signal, and the number “n” of the resonators RSN_1˜RSN_n represents an order of the hairpin band pass filter 20. Therefore, a designer can vary the number “n” according to different demands. For example, FIG. 3 is a frequency response diagram of the hairpin band pass filter 20 when n=5. In FIG. 3, curves a1, b1 and c1 are respectively corresponding to scattering parameters S11, S21 and S22. Since a related definition is fairly known for people in the art, a detail description is omitted herein, and can be found in books listed below, for example, Microelectronic Circuits, 2004, 5th edition, written by Adel S Sedra. and Kenneth C. Smith, Feedback Control of Dynamic Systems, 1994, 3rd edition, written by Gene F. Franklin, J. David Powell and Abbas Emami-Naeini, and Nonlinear Microwave Circuit, 1998, written by Stephen A Maas. As can be seen from FIG. 3, the insertion loss of the desired lowest frequency 18.3 GHz is 5 dB, and a lowest insertion loss of the image frequency section 17.3˜17.8 GHz is 40.3 dB. Therefore, the image frequency rejection ratio is 40.3−5=35.3 dB.
Generally, as the order of the hairpin band pass filter 20, or the number “n”, is getting higher, the rejecting effect of image frequency is getting better. However, the circuit area is also getting larger, and thus, increases cost. On the contrary, reducing the size and limiting the order of the hairpin band pass filter 20, the rejecting effect of image frequency may cause an insufficient condition, and influences the quality of signal receiving.